The level of immunoglobulin gene V-J recombination activity in cell lines derived from lymphoid or nonlymphoid lineages was examined. The assay uses an extrachromosomal DNA substrate, and thus avoids difficulties associated with the use of chromosomally integrated DNA. The recombination activity is found to vary progressively during B lymphoid development. It is low at a very early stage (pro-GMB), much higher in pro-B cells, but then declines through later stages, reaching an undetectable level in mature B cells. The activity is also present in multi potential progenitors of myeloid cells, and in pre-T but not mature T cells. No activity was found in several nonhematopoietic cell lines.